A control circuit in a conventional air blowing device available as a ventilation device is configured as described below. Alternating current (AC) power supplied from an AC power supply is first converted by an AC-direct current (DC) conversion circuit from an alternating current into a DC, and then the DC is smoothed by a capacitor. A DC voltage generated at both ends of the capacitor is input via a current detector into an inverter. Upon the input of DC voltage, the inverter causes six semiconductors configuring the inverter to mutually operate (switch) to drive a motor. Since a current flowing into the motor flows via the inverter and the current detector, a voltage induced on both ends of the current detector is detected.
The motor is attached with a position sensor that generates a signal corresponding to a position of a rotor that has moved as the motor rotates. Based on a signal from the position sensor, a rotation speed detection unit detects a rotation speed of the motor. The rotation speed detected at this time is output to an air volume computation unit and a speed controller. Based on a current value of the motor, which is detected by the current detection unit, and a rotation speed of the motor, which is detected by the rotation speed detection unit, the air volume computation unit computes a volume of air generated by a fan coupled to the motor.
A deviation between an air volume computed by a target rotation speed computing unit and a target air volume is further measured to compute a target rotation speed toward which the motor operates so that the deviation becomes 0. The speed controller controls a speed of the motor toward the target rotation speed computed by the air volume computation unit. Therefore, since a volume of air generated by the fan coupled to the motor can reach the target air volume, a configuration of constantly controlling an air volume has been achieved (for example, see Patent Literature 1).